Laser scanning projectors constructed from microelectromechanical system (MEMS) components can be relatively small, and therefore implemented into easily portable devices such as picoprojectors, wearable devices, lidar, and smart headlamps. These laser scanning projectors can be used to display fixed or moving video images on a screen, wall, lens (in the case of a smartglass wearable), or user's skin (in the case of a smartwatch wearable). Since modern digital media is often in a high definition format, the quality of image reproduction of such a laser scanning projector is of importance in a commercial environment.
Some parameters defining image quality are image sharpness (which determines the amount of details that a single image may convey), noise level (which is a random variation of image density, visible as grains in the image and pixel level variations in digital images), contrast (which is the slope of the tonal response curve), and distortion (i.e. an aberration that causes straight lines to curve near the edges of images).
The larger the image is, the more challenging it may be to display it at a high quality, according to the parameters defined above. This is a challenge that laser scanning projector manufacturers encounter. However, with MEMS based laser scanning projectors, this challenge may be substantially increased. The MEMS laser scanning projectors may use a very small, complex, fragile scanning mirror architecture that is based on a modulated laser source and reflective mirror mechanisms. In addition, there are several additional problems associated with MEMS laser projectors that lead to lowering the image quality.
In general, MEMS laser scanning projectors function by optically combining red, green, and blue laser beams to form an RGB laser beam, and then directing the RGB laser beam to either a bi-axial mirror, or a set of two uni-axial mirrors working in tandem. The mirror or mirrors are controlled so as to move, or “scan” the laser in a series of vertically spaced apart horizontal lines at a rate of speed such that the human eye perceives a complete image.
The mass of the MEMS mirrors renders it extremely difficult to operate the mirror or mirrors according to step functions. Therefore, the vertical scanning is performed continuously, with a typical resulting scan pattern being shown in FIG. 1A. As shown, the horizontal scan lines are tilted. This may result in the image not being properly displayed—as may be noted, some parts of the image are never reached while others are scanned twice. Therefore, this common scanning method described above may result in a discontinuous image, which is commercially undesirable.
Therefore, extensive research and development has been performed to produce new scanning methods and techniques. One such exemplary scanning technique is described in U.S. Pat. Pub. 2011/0234898 to Goren. In this patent application, a singular RGB laser is scanned in a first scan pattern, shown in FIG. 1B. Once the first scan pattern has been formed, the laser is then scanned in a second scan pattern shown in FIG. 1C. In these scan patterns, each scan line SL0-SL7 is formed by the laser moving from one corner of the scan line to the diagonally opposing corner of that scan line. The scan period between the first and second scan patterns is quick enough such that the human eye perceives a single image, as shown in FIG. 1D. The use of the different scan patterns helps to correct the discontinuous image issues described above. Note that each scan pattern is formed using but one RGB laser.
Due to the mass of the mirror or mirrors and due to the physical limitations in the construction thereof, using the above techniques to generate images with HD or greater than HD resolutions may be extremely difficult—it may not be possible to move mirrors at a rate fast enough to generate the requisite number of scan lines, for example.
Therefore, further development in the area of scanning laser projectors is needed.